What Size Generator Do I Need? (Simple 3-Step Method + Calculator)

Figuring out what size generator you need is fairly simple (in theory): Just add up all the wattages and now you know how big a generator you need. That’s the most common mistake generator sizing calculators make. In practice, there is a lot more you need to consider; namely should consider the ‘starting wattage’.

The best size generator is the smallest size you need. You don’t want a generator that’s too small to handle all the loads, nor do you want a too big generator; that’s just overkill and extra expenses.

Here are the absolute worst things that can happen if you inadequately size your generator:

  • You have an electrical grid power outage, turn on your whole-house generator, and the big electricity-hungry units like heating, furnace, or AC won’t start (despite having sufficient running wattage generator).
  • You arrive at a camping site and you want to start a heater, and the heater doesn’t start because the peak wattage of the heater is too big.

As you might see – and what most generator size calculators omit – is the sufficient running wattage but insufficient starting wattage.

We’ll cover all the details about how to adequately size a generator for home, AC, refrigerator, etc., and show you a simple 3-step rule of thumb method (a formula, if you will) of how to properly size your generator.

This is the simplest way how to calculate the generator site; we also include the ‘What Size Generator Do I Need For My House’ calculator in the 3rd step of the rule of thumb method. Here’s a screenshot of how this calculator looks like (you will find it further on):

what size generator do i need for my house calculator
Example: A house with a 5,000W total running wattage load and a 3,000W peak wattage unit (AC, fridge, furnace) requires a 5,000W running watts and 8,000W starting watts generator.

Let’s start with an easy example to illustrate how to calculate what size generator you really need:

Quick Example: What Size Generator To Run Refrigerator And Freezer?

A big refrigerator and freezer run on:

  • 700W (starting wattage).
  • Requires 2,200W to start (starting wattage; also known as ‘peak wattage’ or ‘surge wattage’).

Now, a perfect size generator to run this refrigerator/freezer would have 700W running wattage and 2,200W starting wattage, right?

This is what they don’t tell you:

In most cases, it’s impossible to get a generator size that will perfectly match both the watt draw and amp draw. You can only get the closest size of a generator (not exactly, but close). In this case, the most adequate size options are:

  • Honda EU2200ITAG. 2,200W starting wattage and 1,800W running wattage.
  • Westinghouse iGen2200. 2,200W starting wattage and 1,800W running wattage.
  • DuroMax XP2200EH. 2,200W starting wattage and 1,800W running wattage.

Here’s what you may be thinking right now: ‘Why would I need a 1,800W running wattage generator if my fridge + freezer runs on only 700W? A 1,000W generator should be sufficiently sized for that.’

That’s right. A 1,000W generator will be able to run both the refrigerator and freezer but only once the units are turned on. Here’s a crucial part that many generator sizing calculators omit:

You won’t be able to start your fridge + freezer (despite meeting the running watts criteria).

If you can’t start an electric device, you can’t really run it. In the event of a power outage or on a camping site, the biggest mistake you can make when sizing a generator is to choose one with adequate running wattage but too low starting wattage.

We shouldn’t be focused on getting the running wattage right. We should focus, and we can’t stress this enough, getting the starting wattage right.

Generator Starting Watts VS Running Watts

Running watts are the electric power an electric device needs to operate continuously. Usually, we have no problem finding a generator with enough running wattage to power all our devices.

Starting watts are the electric power an electric device needs to start. Just to turn on. In many cases, we do have a problem finding an adequately sized generator with sufficient starting watts.

This starting watts vs running watts requirement is a bother, no doubt. Here’s are some inconvenient truths when it comes to adequately sizing a generator (with starting wattages in mind):

  • Starting wattage is hard to find. It’s easy to check the running wattage – just check the label on the fridge, for example – but figuring out what the peak watts are can be difficult.
  • Some devices require 3-4 times as many watts to start vs watts to run. In our example, we need 2,200W peak watts despite the fridge + freezer running on only 700W.
  • Generators don’t produce 3-4 times more starting wattage than running wattage. As we have seen in the example above, we have 2,200W starting wattage vs 1,800W running wattage. That’s a factor of 1.22 or 22% more starting wattage than running wattage. It’s the same for big whole-house generator sizing. Example: DuroMax XP13000EH has 10,500W running wattage but only 13,000W starting wattage; that’s a 1.24 factor, not the required 3-4 factor.

Here’s what makes sizing a generator a bit easier:

You don’t have to turn on all the devices that the generator should run at the same time. As we’ll see in the 3 step generator sizing calculator below, you should first start a device with the highest starting watts and proceed to turn on lower starting watts devices after that.

3-Step Method Of How To Adequately Size A Generator For House, Camping, You Name It

“What size generator do I need for my house?”

This is the key question most people looking for how big a generator they need have. Here is what most ‘what size generator do I need for my house’ calculators get wrong:

They state that you should just tally up all the running wattages. Let’s say you have a 700W fridge, 1,500W 10,000 BTU window AC, two 250W laptops, 8 60W lights, and a 720W garage door opener. In total that’s 3,900W; you would need a 3,900W generator, right?

Wrong. The 3,900W are just the running wattages. The starting wattages are:

  • Fridge (2,200W).
  • Window AC (2,200W).
  • Two Laptops (2x250W). The laptop’s starting wattage is equal to its running wattage. Basically, you’re powering a laptop battery.
  • 8 Lights (8x60W). Lights also have the same starting and running wattage.
  • Garage Door Opener (1,420W).

If you tally up all the starting wattages, you get a 6,800W starting wattage generator. Do you really need a 6,800W wattage generator?

Not really. The most adequate size of a generator for this house is between 3,900W and 6,800W. How many watts exactly?

Here is where the 3-step rule of thumb method for generator sizing comes in.

The most optimized generator size for this house should have at least 3,900W running wattage and 6,100W starting wattage.

We will go step-by-step to see how we come up with that generator size.

The goal of the generator sizing calculation is to come up with the key parameters:

  1. Generator’s Running Wattage.
  2. Generator’s Starting Wattage.

Let’s start with the 1st step:

1st Step: Make A Wattage Worksheet (For All Simultaneous Run Electric Devices)

Any generator – a whole-house, camping, RV, camper, you name it – should be able to simultaneously run all the electric devices you need.

The first step in figuring out what size generator you need is to make a list of all electric devices you want to run simultaneously.

Note: Simultaneously is the key word here. You won’t use a furnace fan and a portable AC, for example, at the same time. Note down only the electric devices you will run at the same time.

You can put them all on a list of paper or an Excel spreadsheet. Here’s an example:

wattage worksheet for sizing a generator
First, put down all the electric devices you want to run with a generator.

As you can see, we now have all the electric devices we want to run simultaneously. What’s missing are the key parameters: running and starting wattages of each device.

2nd Step: Fill In Running And Starting Wattages For Each Device

This is quite self-explanatory. For each device, you need to fill in how many watts they need to start and how many watts they need to run.

Fridge example: Running wattage = 700W. Starting wattage = 2,200W.

Filling these wattages is quite a task. Finding all the wattages can be quite tedious. That’s why we have prepared a sample table (you will find it below) of average wattages for home appliances, camping appliances, RV and camper electric devices, and so on.

Note: As you will see in the 3rd step, you only need starting wattages for bigger devices such as ACs, furnaces, fridges, and so on. For generator size determination, we will only use the highest starting wattage device.

You can use these to fill in all the wattages for all the electric devices in your wattage worksheet (do note that some electric devices like light bulbs, laptops, TV, and so on, have the same running and starting wattages):

Electric Device: Running Wattage: Starting Wattage: Wattage Ratio:
Fridge/Freezer 700W 2,200W 3.14
Ceiling Fan 60W 130W 2.17
Dehumidifier 240W 240W 1
Heat Pump 4,700W 9,200W 1.96
Space Heater 1,500W 1,500W 1
Dishwasher 1,500W 3,000W 2
Coffee Machine 1,300W 1,500W 1.15
Rice Cooker 200W 700W 3.50
Air Purifier 25W 30W 1.20

You can check more wattages at Honda Generators here.

When you fill up your wattage sheet, you should be looking at something like this:

how big a home generator do i need for my house
Secondly. find all the running wattages and starting wattages and write them down.

Now we have to apply the key step in this rule of thumb generator sizing method:

#3 Sum All Running Wattages And Add The Highest Starting Wattage

When you have all the running wattages, you only need to sum them up. This will already give you the first key parameter for the size of generator you need: the running wattage.

Here is our example:

adding all the wattages a generator should run
1st goal achieved; now you know how many running watts your generator should have.

Now for the tricky part: Determining the starting wattages.

First of all, you need to check which electric device has the highest starting wattages (peak watts). In our case, that’s either the 2,200W starting wattage fridge/freezer or 2,200W starting wattage window AC.

To get how many starting wattage generators you need, you calculate the sum of total running wattages and the highest starting wattage like this:

Required Starting Wattage = All Running Wattages + Highest Starting Wattage

In our case, this looks like this:

Required Starting Wattage = 3,900W + 2,200W = 6,100W

how to determin starting watts for generator
2nd goal achieved; now we know how many starting watts should our generator have.

Hence, we have adequately calculated both of our goals:

  1. We know we need a generator with at least 3,900W running wattage.
  2. We know we need a generator with at least 6,100W starting wattage.

If you need a bit of  help with this final calculation, you can use this calculator:

What Size Generator Do I Need For My House Calculator

Just insert the total running watts and the highest starting watts, and the generator sizing calculator will dynamically calculate how big a generator you need:

0.00 Watts

Generator Starting Wattage Requirement

 

Note: This rule of thumb generator sizing method primarily estimates this most difficult part to figure out – how many peak watts should a generator have. When using this size generator, you should first start the highest start-up wattage device (usually AC or furnace) and then turn on all the rest.

Outtake On Adequate Generator Sizing

The key goal of how to size a generator is to figure out the running and starting wattages. When you have done that, you can choose a generator that matches (or goes beyond) that wattages the closest.

The important thing is that when a power outage strikes, you are prepared with a well-sized generator.

If you have any questions regarding generator sizing, you can explain your situation in the comments below and we’ll do our best to help you out.

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